The present disclosure is related generally to various devices and techniques for sensing and detecting an event. More particularly, the present disclosure is related to ingestible identifiers that employ electromagnetic energy to transmit a signal representative of a sensing or detection event.
Ingestible event markers that include electronic circuitry have been proposed for use in a variety of different medical applications, including both diagnostic and therapeutic applications. State of the art techniques for detecting an ingestible identifier includes making wet contact to two points on the skin and measuring the voltage difference induced by the ingestible identifier activation. Weak signal levels and strong background noise limit these conventional techniques, making detecting and decoding signals from the ingestible identifier difficult and computationally intensive. Two other limitations make communication between an ingestible sensor and an external detector unusual. First is that due to the very small amount of power available on an ingestible sensor and the small size of the ingestible sensor, communication is only one way. There are no aknowledgements returned to sender, as is typical of virtually all duplex communication systems that are prevalent throughout the world. Second, is that due to the small size, limited list of safe materials that may be ingested, and very low manufacturing cost required of this application, it is not commercially feasible—and perhaps not technically feasible but at least would be extremely difficult—to add a crystal oscillator to the circuit. Thus, an inherent distinguishing feature of this communication situation is the uncertainty of the transmitted frequency. Whereas most commercial communication systems operate in an environment where the frequency is known to tens of parts per million, an ingestion sensor powered by a partial power source and stomach fluids is challenged to produce a center frequency with a +/−1% range. Thus, an important contribution of aspects of the present disclosure is the realization of a communication protocol for RF systems where the transmitting power is very low compared to the background noise of the detector and the transmitting frequency uncertainty is large compared to typical modern systems. Compared to other RF systems, ingestible sensors have extremely limited size available for both the coils that transmit the signals and any capacitors that might be used to store energy between communications. In addition, health concerns and opinions of regulatory agencies such as the FDA limit the amount of certain metals that may be digested by a patient, thus placing a cap on the total available power for both sensing and communication. These communication protocols effectively improve the signal levels available for external detection and decoding. There is incentive to increase the signal levels received from ingestible identifiers such that the ingestible identifier can be detected more readily, and by receivers placed on various parts of the body, or worn by a patient.